Alkali metal-promoted zinc titanate hydrogel catalyst for dehydrogenation of organic compounds

ABSTRACT

The catalytic dehydrogenation of at least one dehydrogenatable organic compound which has at least one ##STR1## grouping is carried out in the presence of a zinc titanate hydrogel. The selectivity of the zinc titanate hydrogel may be improved by adding a promoter selected from the group consisting of lithium, sodium, potassium, rubidium and cesium.

This application is a division of application Ser. No. 468,019 filedFeb. 18, 1983, now U.S. Pat. No. 4,463,213.

This invention relates to an improved catalytic process for thedehydrogenation of organic compounds and a catalyst therefor.

Dehydrogenation processes for the conversion of organic compounds tocompounds having a higher degree of unsaturation are well known. U.S.Pat. No. 4,144,277 teaches that zinc titanate is useful as adehydrogenation catalyst in the dehydrogenation of organic compounds.U.S. Pat. Nos. 4,176,140 and 4,327,238 teach that various promoters canbe used with the zinc titanate of U.S. Pat. No. 4,144,277.

While the zinc titanate catalyst of the above referenced patents is anexcellent catalyst for dehydrogenation, the selectivity of the catalystdeclines rapidly as a function of time. While the zinc titanate catalystcan be regenerated to restore its selectivity, it would be desirable toimprove the zinc titanate catalyst in such a manner that selectivitydoes not decline as rapidly as a function of time and thus longerreaction periods can be used before regeneration is necessary. It isthus an object of this invention to decrease the rate at which theselectivity of the zinc titanate catalyst of the above referencedpatents declines as a function of time and thus provide an improvedprocess for the dehydrogenation of organic compounds where longerreaction times are desired.

In accordance with the present invention, a zinc titanate catalyst suchas that taught by the above referenced patents is mixed with a hydrosolof a suitable acidic material. A suitable base is then added to theresulting mixture to form a hydrogel. The hydrogel is dried slowly andcalcined to form what will be referred to as a zinc titanate hydrogel. Apromoter selected from the group consisting of lithium,sodium,potassium, rubidium and cesium may then be added to the zinc titanatehydrogel to improve the performance of the zinc titanate hydrogel.

Once the zinc titanate hydrogel (either promoted or unpromoted) has beenprepared, the zinc titanate hydrogel is utilized as a catalyst in aadehydrogenation process in which organic compounds are dehydrogenated inthe presence of the catalyst to produce organic compounds having ahigher degree of unsaturation.

The dehydrogenation process is preferably carried out in cyclesconsisting of a reaction period and a regeneration period for thecatalyst. The reaction period comprises contacting a dehydrogenatableorganic compound with the catalyst under suitable dehydrogenationconditions in the substantial absence of free oxygen to convert thedehydrogenatable organic compounds to compounds having a higher degreeof unsaturation. After the reaction period, a free oxygen containing gasis passed in contact with the catalyst to regenerate the catalyst byburning off carbonaceous materials which may have formed on thecatalyst.

The use of the zinc titanate hydrogel results in a decrease in the rateat which the selectivity of the zinc titanate catalyst of the abovereferenced patents declines as a function of time. This enables thereaction period for the dehydrogenation process to be increased sinceregeneration is not necessary as often. Also, the use of the promoterimproves the selectivity of the zinc titanate hydrogen especially earlyin the reaction period.

Other objects and advantages of the invention will be apparent from theforegoing brief description of the invention and the appended claims aswell as the detailed description of the invention which follows.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is briefly described as follows:

FIG. 1 is a plot of the data from Example 2 which indicate theimprovement provided by the present invention for a long reactionperiod.

The organic feedstocks which can be dehydrogenated in accordance withthe present invention are dehydrogenatable organic compounds having from2 to 12 carbon atoms per molecule and characterized by having at leastone ##STR2## grouping, i.e., adjacent carbon atoms, each having at leastone hydrogen atom. Suitable compounds includes paraffins, olefins,cycloaliphatics and alkyl aromatic compounds having from 2 to 12 carbonatoms per molecule. Particularly suitable are paraffins having from 2 to5 carbon atoms per molecule and monoolefins having from 4 to 5 carbonatoms per molecule, branched or unbranched. Some examples of suitablehydrocarbon feedstocks are ethane, propane, butane, isobutane, pentane,isopentane, hexane, 2-methylhexane, n-octane, n-dodecane, 1-butene,2-butene, 2-methyl--butene, 2-methyl-2-butene, 2-hexene, 1-octene,3-methyl-4-nonene, 1-dodecene, cyclohexane, and the like and mixtures ofany two or more thereof. Particularly appropriate is the conversion ofethane to ethylene, propane to propylene, butanes to butenes andbutadiene, butenes to butadiene, and isopentane to isoamylenes andisoprene.

The dehydrogenation catalyst employed in the process of the presentinvention is zinc titanate in a hydrogel of a suitable acidic material.In general, the catalyst composition is prepared by first preparing zinctitanate which is then reduced to a small size. The resulting materialis mixed with a hydrosol of a suitable acidic material. A suitable baseis then added to the mixture to form a hydrogel. The resulting hydrogelis dried slowly and calcined to form the zinc titanate hydrogel catalystof the present invention. A promoter selected from the group comprisinglithium, sodium, potassium, rubidium and cesium may be added to the zinctitanate hydrogel as will be more fully described hereinafter.

The zinc titanate portion of the catalyst composition may be prepared byintimately mixing suitable portions of zinc oxide and titanium dioxide,preferably in a liquid such as water, and calcining the mixture in thepresence of free oxygen at a temperature in the range of about 650° C.to about 1050° C., preferably in the range of about 675° C. to about975° C., to form zinc titanate. A calcining temperature in the range ofabout 800° C. to about 850° C. is most preferred because the surfacearea of the zinc titanate is maximized in this temperature range, thusproducing a more active zinc titanate. The titanium dioxide used inpreparing the zinc titanate preferably has extremely fine particle sizeto promote intimate mixing of the zinc oxide and titanium dioxide. Thisproduces a rapid reaction of the zinc oxide and titanium dioxide whichresults in a more active zinc titanate. Preferably the titanium dioxidehas an average particle size of less than 1000 millimicrons and morepreferably less than 30 millimicrons. Flame hydrolyzed titanium dioxidehas extremely small particle size and is particularly preferred inpreparing the zinc titanate. The atomic ratio of zinc to titanium can beany suitable ratio. The atomic ratio of zinc to titanium will generallylie in the range of about 1:1 to about 3:1 and will preferably lie inthe range of about 1.7:1 to about 2.1:1 because the activity of the zinctitanate is greatest for atomic ratios of zinc to titanium in thisrange. The term "zinc titanate" is used regardless of the atomic ratioof zinc to titanium.

The zinc titanate portion of the catalyst composition may also beprepared by coprecipitation from aqueous solutions of a zinc compoundand a titanium compound. The aqueous solutions are mixed together andthe hydroxides are precipitated by the addition of ammonium hydroxide.The precipitate is then washed, dried and calcined, as described in thepreceding paragraph, to form zinc titanate. This method of preparationis less preferred than the mixing method because the zinc titanateprepared by the coprecipitation method is softer than the zinc titanateprepared by the mixing method.

The resulting zinc titanate is reduced to a suitable size for mixingwith a hydrosol of an acidic material by any suitable method such astreatment in an ultrasonic disrupter. The zinc titanate may be reducedto any suitable size with a particle size in the range of about 1 toabout 5 microns being preferred.

The resulting zinc titanate having a fine particle size is mixed with ahydrosol of a suitable acidic carrier. Any suitable acidic carrier suchas an alumina, a silica-alumina or a zeolite material may be utilized.An alumina is preferred because it forms a well dispersed hydrosolphase. Alumina hydrate is particularly preferred because a hydrosol ofalumina hydrate is readily converted to a hydrogel and then to the oxidephase after calcination.

After the zinc titanate has been thoroughly mixed into the hydrosol, asuitable base is added to convert the hydrosol to a hydrogel. Anysuitable base such as alkali metal hydroxides, ammonium hydroxide, orurea may be utilized. Ammonium hydroxide is the preferred base becauseit does not have any metallic component that would remain in thehydrogel.

The resulting hydrogel is dried slowly so that water will not be removedso rapidly that the hydrogel structure will collapse which would resultin excessive loss of pore volume and surface area of the finished zinctitanate hydrogel. Any suitable drying time can be utilized which doesnot result in too rapid removal of water. Preferably, the drying time isin the range of about 8 hours to about 24 hours.

Any suitable temperature can be utilized for the drying of the zinctitanate hydrogel but again the temperature should be such that toorapid removal of water does not result. The temperature is preferably inthe range of about 35° C. to about 150° C. The most preferred dryingcondition is to start the drying process at about 80° C. and increasethe temperature slowly to about 120° C. during the drying time.

After the zinc titanate hydrogel has been dried, the zinc titanatehydrogel is calcined in the presence of free oxygen. Any suitable freeoxygen-containing gas may be utilized with air being preferred becauseof its availability. Also, any suitable time and temperature for thecalcining may be utilized with a preferred time being about two hoursand a preferred temperature being in the range of about 480° C. to about600° C. Although the dried zinc titanate hydrogel can be placed directlyinto a preheated furnace or kiln for calcining, it is preferable for thecatalyst to attain its final temperature during a heating period ofabout two hours.

The finished catalyst composition can contain any suitable weightpercent of zinc titanate. In general, the amount of zinc titanate in thefinished catalyst composition will be in the range of from about 10weight percent to about 50 weight percent based on the total weight ofthe catalyst composition and will more preferably be in the range offrom about 20 weight percent to about 40 weight percent based on theweight of the total catalyst composition.

The unpromoted zinc titanate hydrogel exhibits very poor selectivityearly in the reaction. However, the selectivity of the zinc titanatehydrogel does improve rapidly and once the selectivity does improve thisselectivity is maintained for a much longer period of time than the zinctitanate catalyst of the referenced patents.

The initial performance of the zinc titanate hydrogel can be improved byincorporating a promoter selected from the group consisting of lithium,sodium, potassium, rubidium and cesium. Also, the selectivity of thezinc titanate hydrogel is improved by the addition of the promoter.

The promoter is preferably added to the zinc titanate hydrogel byimpregnation after the zinc titanate hydrogel has been calcined. Thepromoter may be added by any suitable method. Preferably, compounds ofthe promoter are dissolved in a suitable solvent (usually water) and theresulting solution is poured onto the zinc titanate hydrogel. Thesolvent is evaporated and the catalyst is dried and calcined at atemperature suitable to convert the promoter to an oxide. This techniqueutilized is generally referred to as incipient wetness impregnation inwhich the volume of the solution used to add the promoter to the zinctitanate hydrogel is essentially equal to the pore volume of the zinctitanate hydrogel.

The concentration of the promoter expressed as an element can be anysuitable concentration. The concentration of the promoter expressed asan element will generally be in the range of about 0.3 to about 3 weightpercent based on the weight of the zinc titanate hydrogel prior totreatment with the promoter and will preferably be in the range of about0.5 to about 1.5 weight percent based on the weight of the zinc titanatehydrogel prior to treatment.

Any suitable temperature can be utilized to dry the impregnated zinctitanate hydrogel and to calcine the impregnated zinc titanate hydrogel.A suitable drying temperature is in the range of about 80 to about 150C. and a suitable calcining temperature is in the range of about 525 toabout 650 C. Calcination again take place in the presence of freeoxygen.

Any suitable compound of the promoter may be utilized to add thepromoter to the zinc titanate hydrogel. Compounds which are directlyconvertible to the oxide are preferred. These compounds includehydroxides, carbonates, nitrates, acetates and salts of other carboxylicacids. Sulphur containing compounds such as sulfates can be used but areless preferred. Halogen containing compounds should be avoided. Of thepromoters, lithium and sodium are the most preferred.

The dehydrogenation process of this invention can be carried out bymeans of any apparatus whereby there is achieved an alternate contact ofthe catalyst with the dehydrogenatable organic compound and thereafterof the catalyst with the oxygen-containing gaseous phase, the processbeing in no way limited to the use of a particular apparatus. Theprocess of this invention can be carried out using a fixed catalyst bed,fluidized catalyst bed or moving catalyst bed. Presently preferred is afixed catalyst bed.

In order to avoid any casual mixing of the organic compound and oxygen,provision is preferably made for terminating the flow of feed to thereactor and subsequently injecting an inert purging fluid such asnitrogen, carbon dioxide or steam. Any suitable purge time can beutilized. The purge duration will generally range from about 1 minute toabout 10 minutes and will more preferably range from about 3 minutes toabout 6 minutes. Any suitable flow rate of the purge fluid may beutilized. Presently preferred is a purge fluid flow rate in the range ofabout 800 GHSV to about 1200 GHSV.

Any suitable dehydrogenation reaction time may be used in thedehydrogenation process. The dehydrogenation reaction time willgenerally be in the range of about 1 second to about 24 hours and willpreferably be in the range of about 30 minutes to about 8 hours.

Any suitable time for the regeneration of the dehydrogenation catalystcan be utilized. The time for the regeneration of the dehydrogenationcatalyst will generally range from about 1 to about 10 times thereaction period. However, regeneration for a time longer than one hourwill generally not be necessary for any reaction period.

Any suitable catalytic dehydrogenation temperature can be employed whichprovides the desired degree of catalytic activity in the dehydrogenationof the organic feedstock. The dehydrogenation temperature will generallybe in the range of about 426° C. to about 705° C. and will morepreferably be in the range of about 538° C. to about 677° C.

The catalytic dehydrogenation process can be carried out at any suitablepressure. The pressure of the dehydrogenation reaction will generallyrange from about 0.05 to about 250 psia.

Any suitable feed rate for the organic feedstock can be utilized. Theorganic feedstock feed rate will generally be in the range of about 50to about 5,000 volumes of gaseous feedstock per volume of catalyst perhour and will preferably be in the range of about 100 to about 2500volume of gaseous feedstock per volume of catalyst per hour.

A gaseous diluent, such as hydrogen or carbon dioxide, can also beutilized. If the gaseous diluent is utilized, the diluent to hydrocarbonmolar ratio used will generally be in the range of about 0.1:1 to about20:1.

The amount of free oxygen, from any source, supplied during theregeneration step will be an amount sufficient to remove substantiallyall carbonaceous materials from the catalyst. The regeneration step isconducted at the same temperature and pressure recited for thedehydrogenation step although somewhat higher temperatures can be used,if desired.

The operating cycle for the dehydrogenation and regeneration processwill generally include the successive steps of:

(1) contacting a dehydrogenatable organic compound with thedehydrogenation catalyst;

(2) terminating the flow of the dehydrogenatable organic compound;

(3) optionally, purging the catalyst with an inert fluid;

(4) contacting the dehydrogenation catalyst with free oxygen;

(5) terminating the flow of the free oxygen; and

(6) optionally, purging the catalyst with an inert fluid beforerepeating step (1).

EXAMPLE 1

Zinc titanate was prepared by mixing Mallinckrodt powdered zinc oxideand Cab-O-Ti titanium dioxide (flame hydrolyzed) by slurrying in 150 mLof water in a blender for 5 minutes. The ratio of zinc oxide to titaniumdioxide was such as to give an atomic ratio of zinc:titanium in thefinished preparation of 1.8:1. The resulting slurry was dried in an ovenat 105° C. and then calcined in air for three hours at 816° C. Aftercooling, the thus calcined material was crushed and screened, and a-16+40 mesh fraction was reserved for testing. A portion of the thusprepared zinc titanate was utilized as a control catalyst and isreferred to hereinafter as Catalyst A.

The thus prepared powdered zinc titanate (54.1 grams) was slurried into500 mL of water and treated with the transducer of an ultrasonic celldisrupter at high power for about 10 minutes to reduce the particle sizeof the zinc titanate to about 2-10 microns. The resulting slurry wascombined with a suspension of 216 grams of alpha alumina monohydrate andabout 900 mL of water. Sufficient nitric acid was added to the resultingmixture to lower the pH of the resulting mixture from about 7.6 to about3.0 to produce the hydrosol. Ten mL of concentrated ammonium hydroxidewas then added to the hydrosol to produce a hydrogel. The hydrogel wasthen dried in an oven for 18 hours at 82° C. and then the temperaturewas increased to 149° C. for 2 more hours. The thus dried hydrogel wasthen calcined in air in a furnace which was heated to 648° C. during 2hours and then held at that temperature for 2 hours. The resulting zinctitanate hydrogel contained 20 weight percent zinc titanate based on theweight of the total hydrogel. The hydrogel was crushed and screened anda -16+40 mesh fraction was reserved for testing. This catalyst isreferred to hereinafter as Catalyst B.

A second zinc titanate hydrogel was prepared in accordance with theabove procedure except that the quantity of zinc titanate utilized was108 grams and the quantity of alpha aluminum monohydrate utilized was162 grams. The resulting zinc titanate hydrogel contained 40 weightpercent zinc titanate based on the weight of the total hydrogel. Thiscatalyst is referred to hereinafter as Catalyst C.

Catalysts B and C were impregnated with aqueous solutions of lithium orsodium as shown in Table I. The concentrations of the lithium and sodiumwere such as to give the concentrations in the finished catalystcomposition stated in Table I. The stated concentrations were determinedby chemical analysis.

                  TABLE I                                                         ______________________________________                                                 Catalyst  Conc. of Alkali                                            Catalyst Base      Metal, wt. % Form of Alkali                                ______________________________________                                        D        B         Na, 0.1      NaOH                                          E        B         Na, 0.6      NaOH                                          F        C         Na, 1.0      NaOH                                          G        B         Na, 1.3      NaOH                                          H        B         Li, 0.5      LiOH                                          I        B         Li, 0.5      LiOH                                          J        C         Li, 0.5      LiNO.sub.3                                    K        C         Li, 1.4      LiNO.sub.3                                    ______________________________________                                    

Catalysts C-K were used in runs to dehydrogenate isobutane. The runswere made using -16+40 mesh catalyst in a quartz tubular reactor mountedvertically in a temperature controlled tube furance. All runs were madeat 625° C. and atmospheric pressure. The isobutane feed rate was 600GHSV. Snap samples of reactor effluent were taken periodically andanalyzed by GLC. Results of runs using these catalysts are presented inTable II.

                  TABLE II                                                        ______________________________________                                        Catalyst                                                                              Time, sec.                                                                              i-C.sub.4 H.sub.10 conv., %                                                                 Sel. to i-C.sub.4 H.sub.8,                    ______________________________________                                                                        %                                             C       40        98.3          0                                             C       70        87.8          27.1                                          C       160       75.9          47.7                                          D       20        84.8          27.0                                          E       40        56.2          96.6                                          E       100       52.5          95.9                                          E       160       48.6          96.3                                          F       40        58.2          89.2                                          F       100       55.2          92.0                                          G       40        47.7          95.3                                          G       160       40.0          96.1                                          H       20        48.7          86.5                                          H       200       48.9          88.9                                          I       20        24.0          80.1                                          I       200       23.3          81.2                                          J       40        53.7          92.9                                          J       100       51.4          94.6                                          K       40        30.8          80.2                                          K       100       25.8          83.0                                          ______________________________________                                    

Catalyst C, although showing high initial activity, had very poorselectivity for dehydrogenation. Also Catalyst D with 0.1 weight percentNa had relatively poor selectivity. Catalysts E-K all exhibited muchhigher selectivity to produce isobutene from isobutane.

EXAMPLE 2

Runs were made for longer time periods using Catalysts A, C and J andthe procedure of Example 1. All runs were again made at 625° C. andatmospheric pressure. The isobutane feed rate was as set forth inFIG. 1. Again, snap samples of reactor effluent at the times noted inFIG. 1 in which the results of the runs are presented.

Referring to FIG. 1, it can be seen that at the same isobutane feedrate, the promoted zinc titanate hydrogel exhibited a substantiallyimproved yield (selectivity times conversion) especially in the earlypart of the reaction period. The initial yield for the zinc titanatewhich was not in the hydrogel form was high but this yield rapidlydeclined. It can thus be seen that substantially longer reaction periodscan be maintained using the zinc titanate hydrogel and especially usingthe promoted zinc titanate hydrogel. The unpromoted zinc titanatehydrogel is not preferred because of its low initial activity but itcould be utilized if the low initial activity was considered an adequatetradeoff for the fact that the unpromoted zinc titanate hydrogel willmaintain activity for a longer period of time that the zinc titanatewhich is not in the hydrogel form.

Reasonable variation and modifications are possible within the scope ofthe disclosure and the appended claims to the invention.

That which is claimed is:
 1. A catalyst composition comprising ahydrogel of zinc titanate and a suitable acidic carrier and a promoterselected from the group consisting of lithium, sodium, potassium,rubidium and cesium.
 2. A composition in accordance with claim 1 whereinthe concentration of zinc titanate in said hydrogel is in the range ofabout 10 to about 50 weight percent based on the weight of saidhydrogel.
 3. A composition in accordance with claim 1 wherein theconcentration of zinc titanate in said hydrogel is in the range of abut20 to about 40 weight percent based on the weight of said hydrogel.
 4. Acomposition in accordance with claim 1 wherein said suitable acidiccarrier is alumina.
 5. A composition in accordance with claim 1 whereinsaid zinc titanate is prepared by calcining a mixture of zinc oxide andtitanium dioxide in the presence of free oxygen at a temperature in therange of about 650° C. to about 1050° C.
 6. A composition in accordancewith claim 1 wherein the atomic ratio of zinc to titanium in saidhydrogel is in the range of about 1:1 to about 3:1.
 7. A composition inaccordance with claim 1 wherein the atomic ratio of zinc to titanium insaid hydrogel is in the range of about 1.7:1 to about 2.1:1.
 8. Acomposition in accordance with claim 1 wherein said hydrogel is formedby mixing powdered zinc titanate with a hydrosol of alumina hydrate toform a zinc titanate/alumina hydrosol, adding ammonium hydroxide toconvert said hydrosol to a hydrogel and drying and calcining saidhydrogel.
 9. A composition in accordance with claim 8 wherein saidhydrogel is dried for a time in the range of about 8 to about 24 hoursand at a temperature in the range of about 35° C. to about 150° C.
 10. Acomposition in accordance with claim 9 wherein the dried hydrogel iscalcined in the presence of free oxygen at a temperature in the range ofabout 480° C. to about 600° C. for a time of about 2 hours.
 11. Acomposition in accordance with claim 1 wherein said promoter is lithium.12. A composition in accordance with claim 1 wherein said promoter issodium.
 13. A composition in accordance with claim 1 wherein theconcentration of said promoter expressed as an element is in the rangeof about 0.3 to about 3 weight percent based on the weight of saidhydrogel.
 14. A composition in accordance with claim 1 wherein theconcentration of said promoter expressed as an element is in the rangeof about 0.5 to about 1.5 weight percent based on the weight of saidhydrogel.